Phase transformations in plasma source ion nitrided austenitic stainless steel at low temperature

Abstract

Plasma source ion nitriding has emerged as a low-temperature, low-pressure nitriding approach for low-energy implanting nitrogen ions and then diffusing them into steel and other alloys. In this work, 1Cr18Ni9Ti (18–8 type) austenitic stainless steel was treated at a process temperature from 280 to 480 °C under an average nitrogen implantation dose rate (nitrogen ion current density) of 0.44–0.63 mA cm⁻² during a nitriding period of 4 h. The nitrided surfaces of the stainless steel were characterized using Auger electron spectroscopy, electron probe microanalysis, glancing angle X-ray diffraction, and transmission electron microscopy. Below 300 °C, a high nitrogen f.c.c. phase (γ_N) and an ordered f.c.c. phase (γ′) mixed phase and a γ_N and a nitrogen-induced martensite (ɛ′_N) mixed phase were obtained respectively under lower and higher nitrogen implantation dose rates. In the range of 300–450 °C a single γ_N phase was observed under various nitrogen implantation dose rates. Above 450 °C, the decomposition of the γ_N phase to a CrN phase with a b.c.c. martensite was obtained. Phase states and phase transformations in the plasma source ion nitrided 1Cr18Ni9Ti stainless steel at the low process temperatures are dependent on all the process parameters, including process temperature, nitrogen implantation dose rate, nitrogen ion energy, and processing time, etc.. The process parameters have significant effects on the formation and transformation of the various phases.